Cardiac therapy method with duration timer

ABSTRACT

A cardiac therapy method with duration timer is disclosed, using an implanted cardiac pulse generator. A patient&#39;s heartbeat is sensed and the intervals between heartbeats are averaged. The number of temporary storage bins, in the form of RAM locations, are provided, including a sinus bin, a low rate tachycardia bin, a high rate tachycardia bin, and a fibrillation bin. The storage bin corresponding to the cardiac rhythm band of the determined average heartbeat interval is incremented. A maximum count limit is assigned to each storage bin. When the first bin reaches its maximum count limit, a diagnosis of the patient&#39;s cardiac rhythm is provided. Upon detection of a tachycardia, a duration timer is started and the tachycardia is treated in accordance with a programmed routine. If sinus is detected during the predetermined time period, then the duration timer is cleared. If fibrillation is detected during the predetermined time period, then the duration timer is cleared and fibrillation is treated by delivering a high energy shock to the heart.

CROSS-REFERENCE TO RELATED APPLICATION

This application is related to U.S. Application Ser. No. 363,967, filedMay 19, 1989.

FIELD OF THE INVENTION

The present invention concerns a novel method of cardiac therapy usingan implanted cardiac pacer/ defibrillator.

BACKGROUND OF THE INVENTION

Implantable defibrillators are known in the prior art in which thespectrum of heart rates is divided into several distinct bands. Suchbands may include normal sinus rhythm, tachycardia and fibrillation.

We have found that it would be extremely useful to divide the spectrumof heart rates into five bands, including bradycardia, normal sinusrhythm, slow tachycardia, fast tachycardia and fibrillation. However,when rhythms occur that have a combination of intervals, i.e., where therhythm is oscillating around a border between two tachyarrhythmias, theimplanted device must decide which arrhythmia is present. Further, thedevice must detect sinus rhythm when it occurs.

The present invention concerns a combined antitachycardiapacemaker/defibrillator which is implanted to treat potentially lethalarrhythmias. antitachycardia pacemaker/defibrillator which is implantedto treat potentially lethal arrhythmias. However, antitachycardia pacingcan be an unpredictably prolonged therapy, and can result in leaving thepatient in jeopardy if more definitive therapy is not used within ashort period of time. One solution to this problem would be to severelylimit the programability of the antitachycardia pacing feature. However,this is not a desirable solution.

Another potential problem in an automatic tiered therapy defibrillator,is that if a rapid arrhythmia such as a high rate tachycardia orfibrillation is detected, and the associated therapy is delivered, thedevice must know when to stop delivering the therapy. The device couldbe programmed to stop delivering therapy when the original detectioncriteria are no longer met.

Currently available defibrillators have a single rate cutoff. If it isexceeded, therapy is delivered. Normally if the average heart rate ishigh enough to be considered a tachycardia, an automatic device willprovide tachycardia therapy. However, it is possible to have rhythmsthat have an average tachycardia rate but have an alternating pattern ofintervals and should not be treated. These rhythms are sometimes calledbigeminal rhythms. An example of a bigeminal rhythm is 600 msec/300msec/600 msec/300 msec/etc. The average interval is 450 msec. If thetachycardia detection criterion is 500 msec, the device wouldinappropriately diagnose tachycardia.

Some prior art antitachycardia devices require a sequential number ofintervals below the interval criterion for tachycardia. However, thismakes it difficult to detect arrhythmias unless the arrhythmia rate isvery stable.

It is, therefore, an object of the present invention to provide a methodfor determining arrhythmia hierarchy in an implanted defibrillator whichhas a number of distinct bands.

Another object of the present invention provide definitive therapy forpatients suffering from tachycardia, using an implanted combinedantitachycardia pacemaker/defibrillator.

A further object of the present invention is to utilize an arrhythmiadetection method using hysteresis, in which there is one rate cutoff fordeciding to deliver therapy and a separate, lower rate cutoff, fordeciding to cease giving therapy.

A still further object of the present invention is to provide arrhythmiadetection inhibition with low tachycardia rate averages, but intervalalternans. In this manner, the system will keep track of the ratio ofsinus intervals to tachycardia intervals, and will require moretachycardia intervals than sinus intervals in order for an arrhythmia tobe detected. Therefore the presence of a tachycardia can be quicklydetermined, without inappropriately detecting a bigeminal rhythm astachycardia.

Other objects and advantages of the present invention will becomeapparent as the description proceeds.

SUMMARY OF THE INVENTION

In accordance With the present invention, a cardiac therapy method isprovided using an implanted cardiac pulse generator. In one embodiment,the method comprises the steps of: sensing a patient's heartbeat;providing storage means including a plurality of temporary storage bins,each of which corresponds to a different cardiac rhythm band;determining the intervals between heartbeats; incrementing the storagebin corresponding to the cardiac rhythm band of the determined heartbeatinterval weighted by the current coverage; assigning a maximum countlimit to each storage bin; detecting when the first bin reaches itsmaximum count limit; providing a diagnosis of the patient's cardiacrhythm that is responsive to the first bin to reach its maximum countlimit; and initializing the temporary storage bins.

In the illustrative embodiment, the step of providing storage meanscomprises the step of providing a sinus storage bin, a low ratetachycardia storage bin, a high rate tachycardia storage bin, and afibrillation storage bin.

The method of an embodiment in accordance with the principles of thepresent invention also comprises the steps of: sensing a patient'sheartbeat; determining the intervals between heartbeats; upon detectionof an average tachycardia rate, starting a duration timer to time apre-determined time period and treating for tachycardia in accordancewith a programmed routine; if sinus is detected during the predeterminedtime period, then clearing the duration timer; if fibrillation isdetected during the predetermined time period, then clearing theduration timer and treating for fibrillation; and if tachycardiacontinues for the predetermined time period, then commencingfibrillation therapy.

In the illustrative embodiment, the step of determining the intervalsbetween heartbeats includes the step of averaging a selected number ofheartbeats.

The method of an embodiment of the present invention also comprises thesteps of: sensing a patient's heartbeat; determining the intervalsbetween heartbeats; if the heartbeats exceed a first rate, then treatingfor an arrhythmia; and continuing to treat for an arrhythmia unless theheartbeat rate declines to below a second rate, with the second ratebeing lower than the first rate. This allows use of a higher arrhythmiadetection rate in a tiered defibrillator, without the disadvantage offailing to terminate slower arrhythmias that might result from thetherapy.

The method of an embodiment of the present invention also includes thesteps of: sensing a patient's heartbeat, determining the intervalsbetween heartbeats; determining the ratio of sinus intervals toarrhythmia intervals; treating for arrhythmia only if the number ofarrhythmias exceed the number of sinus intervals notwithstanding thatthe average of the sinus and arrhythmia intervals is shorter than sinusrhythm. This prevents the system from inappropriately detecting abigeminal rhythm as an arrhythmia.

A more detailed explanation of the invention is provided in thefollowing description and claims, and as illustrated in the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAwINGS

FIG. 1 is a block diagram of an implantable pacer/defibrillator systemconstructed in accordance with the principles of the present invention.

FIG. 2 is a flow chart of the arrhythmia discrimination method of thesystem of FIG. 1.

FIG. 3 is a flow chart of the bin interval block from the flow chart ofFIG. 2.

FIG. 4 is a flow chart of the determine rhythm block from the flow chartof FIG. 2.

FIG. 5 is a flow chart of the interval alternans diamond from the flowchart of FIG. 2.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENT

Referring to FIG. 1, the block diagram for the implantable defibrillatorincludes four ICs and a set of high voltage discretes. The batteryproduces a positive voltage with respect to ground that varies fromabout 6.4 volts when new, to 5.0 volts at the end of service. Thebattery directly powers IC2 30 and the high voltage discretes 60.

IC2 contains a band-gap reference circuit 31 that produces 1.235 volts,and 3 volt regulator that powers the microprocessor 90, ICl 70, and theECG storage RAM 77 through line 100. The 3 volt regulator runs off of aswitched capacitor V 2/3 battery voltage down converter 33 for improvedefficiency.

The microprocessor 90 communicates with IC2 through a data and addressbus 83 and an on-chip interface 34 that contains chip-select, addressdecoding and data bus logic as is typically used with microprocessorperipherals. The internal bus 35 allows the microprocessor to control ageneral purpose ADC 36, the atrial pace circuits 37, the ventricularpace circuits 38, and the HV control and regulate block 39.

The ADC 36 is used by the microprocessor to measure the battery andother diagnostic voltages within the device.

The atrial pace circuits 37 include a DAC that provides the ability topace at regulated voltages. It communicates with the atrium of a heart40 through two lines. One line 41 is a switchable ground; the other line42 is the pacing cathode and is also the input to the atrial senseamplifier, as will be described below.

The ventricular pace circuits 37 include a DAC that provides the abilityto pace at regulated voltages. It communicates with the ventricle of aheart 40 through two lines. One line 43 is a switchable ground; theother line 44 is the pacing cathode and is also the input to theventricular sense amplifier, as will be described below.

Both the atrial and ventricular pace lines pass through high voltageprotection circuits 45 to keep the defibrillation voltages generated bythe device from damaging the pacing circuits 37 and 38.

The HV control and regulate block 39 on IC2 30 is used by themicroprocessor 90 to charge a high voltage capacitor included in the HVcharge block 46 to a regulated voltage, and then to deliver thedefibrillating pulse to the heart 40 through the action of switches inthe HV delivery block 47. An HV sense line 48 is used by the HVregulation circuits 39 to monitor the defibrillating voltage duringcharging. An HV control bus 49 is used by the HV control circuits 39 tocontrol the switches in the HV delivery block 47 for delivering thedefibrillating pulse to the electrodes 52, 53 through lines 50 and 51.

ICI 70 is another microprocessor peripheral and provides timing,interrupt, telemetry, ECG storage, and sensing functions.

A dual channel electrogram sensing and waveform analysis section 71interfaces with the atrium and ventricle of the heart 40 through lines42 and 44 respectively. The sensed electrogram is amplified anddigitized. The amplifiers contained in this section 71 have multiplegain settings that are under microprocessor control for maintaining anAGC. Features such as peak voltage and complex width are extracted bythe waveform analysis circuits 71 for the microprocessor 90 to use indiscriminating arrhythmias from normal sinus rhythm. The voltagereference 31 from IC2 30 is used by the digitizer circuit 71 in theusual fashion, and is supplied by line 72.

The digitized ECG is provided to the RAM controller 74 through a bus 73.The RAM controller sequences through the addresses of a static RAM 77 tomaintain a pretrigger area, and this produces a post trigger area uponcommand from the microprocessor 90.

The crystal and monitor block 78 has a 100 KHz crystal oscillator thatprovides clocks to the entire system. The monitor is a conventional R-Coscillator that provides a back-up clock if the crystals should fail.

The microprocessor communicates with IC1 through two buses, 83 and 84.One bus 83 is a conventional data and address bus and goes to an on-chipinterface 81 that contains chip select, address decoding and data busdrivers as are typically used with microprocessor peripherals. The otherbus 84 is a control bus. It allows the microprocessor to set up avariety of maskable interrupts for events like timer timeouts, and senseevents. If an interrupt is not masked, and the corresponding eventoccurs, an interrupt is sent from IC1 70 to the microprocessor 90 toalert it of the occurrence. On IC1 70, the up control and interruptsection 79 contains microprocessor controllable timers and interruptlogic.

The device can communicate with the outside world through a telemetryinterface 80. A coil 105 is used in a conventional fashion to transmitand receive pulsed signals. The telemetry circuits 80 decode an incomingbit stream from an external coil 110 and hold the data for subsequentretrieval by the microprocessor 90. When used for transmitting, thecircuit 80 receives data from the microprocessor 90, encodes it, andprovide s the timing to pulse the coil 105. The communication functionis used to retrieve data from the implanted device, and to change themodality of operation if required.

The microprocessor 90 is of conventional architecture comprising an ALU91, a ROM 92, a RAM 93, and interface circuits 94. The ROM 92 containsthe program code that determines the operation of the device. The RAM 93is used to modify the operating characteristics of the device as regardsmodality, pulse widths, pulse amplitudes, and so forth. Diagnostic datais also stored in the RAM for subsequent transmission to the outsideworld. The Arithmetic Logic Unit (ALU) 91 performs the logicaloperations directed by the program code in the ROM.

The program code is written to perform certain desirable functions whichare best described in flowchart form.

The method of arrhythmia discrimination of the present invention isillustrated in the flow chart of FIG. 2. The "bins" referred to thereinar RAM memory locations. The basic technique is to use the average ofthe last four sensed events to help decide how to bin each heartbeatinterval. The first bin to reach its programmed number of countsdetermines the diagnosis.

The following technique rapidly converges on a solution. In the eventthat the arrhythmia is indeterminate, it defaults to the moreconservative diagnosis.

Referring to FIG. 2, the system contains a loop that starts by awaitingthe next R-wave. After the R-wave is detected, the interval of theR-wave is put into a register called INT. A register called AVERAGE isupdated with that interval (AVERAGE is the average of the last fourintervals), and the INT timer is restarted. The system then questionswhether or not the arrhythmia is in progress. If no arrhythmia is inprogress, the pre-therapy detection parameters are used. If thearrhythmia is in progress and therapy has already been delivered forthis arrhythmia, then the post-therapy detection parameters are used.

In this embodiment, the post-therapy detection parameter is effectivelya lower rate cutoff then the pre-therapy detection parameter. Thus if ahigh rate tachycardia is detected, there will be continued treatment forthe tachycardia notwithstanding the fact that it may revert to a lowerrate tachycardia. In other words, the post-therapy detection parameterswill detect what normally would have been a low rate tachycardia as ifit were a high rate tachycardia. Thus even if the arrhythmia slows tothe range that would previously have been a low rate tachycardia, it isconsidered to be a high rate tachycardia episode. In this manner, thetherapies can be allowed to increase in effectiveness rather than goingbackwards in effectiveness.

The system then falls through to the bin interval block. At that time,the system decides whether the interval that it just received belongs inlow rate tachycardia (TACH A), high rate tachycardia (TACH B),fibrillation, or the sinus bin. The bin interval block is described inmore detail below with respect to FIG. 3.

The bin interval block leads to the determine rhythm block where theresults of the binning exit as either having detected sinus, low ratetachycardia, high rate tachycardia, EHR (extended high rate) or havingdetected fibrillation.

If the system detects an arrhythmia while there is a low ratetachycardia AVERAGE, an additional check is made to see if the intervalalternans inhibition is in effect. In this manner, the system looks forthe case where there is a tachycardia/sinus/tachycardia/sinus, etc.sequence of events. If that is the case, the system inhibits deliveringtherapy and waits for the next R-wave. If that is not the case, thentherapy is delivered. After delivering therapy, the system returns towaiting for the next R-wave.

The "determine rhythm" block is described in more detail with respect toFIG. 4, and the interval alternans block is described in more detailbelow with respect to FIG. 5, described below.

In summary with respect to the discrimination flow chart of FIG. 2, thepatient's R-waves are sensed and put into an interval storage registerlabeled INT. There is a short term averaging of the intervals (in thispreferred embodiment the last four intervals are averaged, although afewer or greater number could be used). Depending upon the averageinterval, it can then be determined into which bin the interval shouldbe put.

Now referring to the bin interval flow chart of FIG. 3, the flow chartis entered from the detection hysteresis part of the flow chart of FIG.2, where the system determines whether to use pre-therapy detectionparameters or the post-therapy detection parameters. Those parametersare the rates at which the system detects the tachycardia and the numberof intervals required for detecting tachycardia. The number of intervalsrequired is the depth of the bin. The system first checks to see if theaverage interval (AVERAGE) is longer or shorter than TACH A. If it islonger than TACH A, then there is a sinus average (note that a separateprogrammable interval could be used as a sinus interval criterion, butin the preferred embodiment the TACH A criterion is used). If theinterval is a tachyarrhythmia interval, and there is a sinus AVERAGE, nobin is incremented. If there is a sinus AVERAGE, and there is a sinusinterval then a sinus interval is binned. If the AVERAGE is shorter thanTACH A, then a tachyarrhythmia may be diagnosed.

Next, the system checks whether the interval is shorter than low ratetachycardia (TACH A). If it is shorter, then the interval is either atachycardia or fibrillation interval. If the interval is longer than lowrate tachycardia (TACH A), then it is a sinus interval and the systemexits.

If the interval is shorter than TACH A, the average interval (AvERAGE)is checked to see if it is shorter than a low rate tachycardia. If it isnot, there is a sinus average and a sinus interval and the system binsone sinus count. If the average is less than a low rate tachycardia,that means the patient has a tachycardia or fibrillation average, butsince the system detected a sinus interval, nothing is binned. Thesystem just falls through to the exit.

If, at the first decision block, the system determines that the intervalis less than TACH A, then the patient has a tachycardia or afibrillation interval and the system checks to see whether AVERAGE isless than the fibrillation requirement. If AVERAGE is less than thefibrillation criterion, the system bins a fibrillation interval. Thuswhether it is a TACH interval or a TACH A or a TACH B interval, the factthat AVERAGE is fibrillation means that the system will go ahead and binit as a fibrillation interval.

If, on the second decision block through the main decision block tree,the system determines that AVERAGE is not less than the fibrillationinterval, that would mean that AVERAGE is a tachycardia average sincethe system has already determined that it is not a fibrillation average,but the interval may either be a tachycardia interval or a fibrillationinterval. The system then checks to see whether the interval is lessthan the fibrillation interval requirement. If it is, the system binsthe interval as a fibrillation interval. If not, then the system is atthe point where it recognizes that the patient has a tachycardia averageand a tachycardia interval. The system then has to determine whether itis a low rate tachycardia or a high rate tachycardia. To this end, thesystem checks to see if the average is less than the high ratetachycardia interval. If it is, then the patient has a high ratetachycardia average and it is binned as a TACH B. If not, the patienthas a low rate tachycardia average, and the system has to check to seeif the interval is less than the TACH B interval. If it is, then it isbinned as a TACH B interval. If it is not, then it is binned as a TACH Ainterval and exits.

It can be seen that, with respect to the bin interval system, theaverage interval (AVERAGE) is used to determine where to bin theinterval. In addition, the system checks to determine whether theinterval represents sinus, TACH A, TACH B or fibrillation. As differentintervals are detected, the decision blocks place the interval data intodifferent bins depending on the interval determination using AVERAGE.Until a bin is full, the system has not decided what the arrhythmia is.Once the bin is filled, the system then diagnosis sinus rhythm or anarrhythmia and treats for that particular arrhythmia in response to thefilled bin.

In a specific example, the values that the microprocessor uses toimplement arrhythmia detection are as follows. These are fixed valuesthat can be adjusted to change the arrhythmia detection for individualpatients. The ranges and nominal values are shown for information only.

    ______________________________________                                        TACH A INT                                                                              Intervals between this and TACH B INT are                                     used to diagnose TACH A. Intervals longer                                     than this are considered sinus; 450 msec.                           TACH B INT                                                                              Intervals between this and FIB INT are                                        used to diagnose TACH B; 380 msec.                                  FIB INT   Intervals shorter than this are used to                                       diagnose fibrillation; 310 msec.                                    NUM SINUS The number of intervals for sinus                                             detection; 3.                                                       NUM TACH A                                                                              The number of intervals for TACH A                                            detection; 8.                                                       NUM TACH B                                                                              The number of intervals for TACH B                                            detection; 8.                                                       NUM FIB   The number of intervals for fib detection;                                    12.                                                                 ______________________________________                                    

The microprocessor maintains the following registers:

    ______________________________________                                        AVERAGE    The average of the last four intervals.                            SINUS CNT  The number of sinus intervals detected.                            TACH A CNT The number of TACH A intervals detected.                           TACH B CNT The number of TACH B intervals detected.                           FIB CNT    The number of fib intervals detected.                              TS RATIO   Used to avoid detecting a rhythm with                                         interval alternans as an arrhythmia.                               ______________________________________                                    

AVERAGE is the average of the last four ECG intervals and is updated bythe microprocessor every interval. Every interval is compared againstAVERAGE. The intervals are binned by the microprocessor according to thefollowing conditions:

    ______________________________________                                        If     Interval > TACH A INT                                                  and    AVERAGE > TACH A INT                                                   then   Increment SINUS CNT                                                    If     Interval > TACH A INT                                                  and    AVERAGE < TACH A INT                                                   then   No bin is incremented                                                  If     TACH A INT > Interval > TACH B INT                                     and    TACH A > AVERAGE > TACH B INT                                          then   Increment TACH A CNT                                                   If     TACH A INT > Interval > TACH B INT                                     and    TACH B INT > Average > FIB INT                                         then   Increment TACH B CNT                                                   If     TACH A INT > Interval > TACH B INT                                     and    AVERAGE < FIB INT                                                      then   Increment FIB CNT                                                      If     TACH B INT > Interval > FIB INT                                        and    TACH A > AVERAGE > FIB INT                                             then   Increment TACH B CNT                                                   If     TACH B INT > Interval > FIB INT                                        and    AVERAGE < FIB INT                                                      then   Increment FIB CNT                                                      If     Interval < FIB INT                                                     and    TACH A > AVERAGE                                                       then   Increment FIB CNT                                                      ______________________________________                                    

The microprocessor increments the appropriate bin after every detectedECG Interval. The first bin to count up to its respective limit causesthat arrhythmia, or sinus rhythm, to be diagnosed. Upon each diagnosis,the arrhythmia detection bins are initialized.

In the operation of the system, if the system sees mixed TACH A and TACHB intervals is AVERAGE is TACH B, all the intervals are binned as TACHB. Therefore, it there is a high rate tachycardia average, the presenceof some slow intervals will not cause the system to delay deciding on anarrhythmia diagnosis; the arrhythmia interval will still be binned thesame as if they were the faster arrhythmia. If there is a fibrillationaverage, then any TACH interval gets binned as a fibrillation. If thesystem detects a TACH B average, then any TACH A interval gets binned asa TACH B. In this manner, the system defaults to the most conservativediagnosis. For example, if the average is TACH B and a fibrillationinterval is detected, the system will bin a fibrillation interval ratherthan a TACH B interval in order to be as conservative as possible. Insummary, the system always tends to bin diagnose the more seriousarrhythmia.

The following is a discussion of the determine rhythm flow chart of FIG.4. Referring to FIG. 4, once the interval has been placed into a bin,the system must determine whether an arrhythmia has been detected. Ineffect, the system is determining whether a bin is full. However, thesystem must first go through and see if the AVERAGE is less than the EHRdetection interval (in this embodiment TACH A is used as the EHRdetection interval, though generally it can be a separately programmablerate criterion). If it is, then the system checks to see if the EHRtimer is already running. If it isn't, that means that the patient hasgone from either sinus to the beginning of a tachycardia or there hasbeen a defibrillation therapy but there is still a tachycardia inprogress. If either one of those are true, then the extended high rate(EHR) timer is started. If the EHR timer has been ongoing, the systemjust falls through and checks to see if the EHR timer has timed out. Ifit has, the EHR timer is cleared and the diagnosis is that EHR is thearrhythmia that has been detected.

If the EHR timer has not timed out, the system falls through and checksto see if any of the bins are full. If none of the bins is full, thesystem is either redetecting sinus or is redetecting one of thearrhythmias; the system goes to the sinus/redetecting exit. If a bin isfull, since only one bin will be full at this time, the system checks tosee which bin is filled and then clears all of the bins.

If the system detects that either a sinus bin is full or a fibrillationbin is full, then the EHR timer is cleared and the diagnosis is eithersinus or fibrillation, respectively. If the system determines either aTACH A or a TACH B, the EHR timer is not cleared and the diagnosis isTACH A or TACH B, respectively. The depth of a bin is determined inmemory by how the device is programmed. Thus, each bin is a RAM locationthat is counting up to a value that is determined by another RAMlocation that is under programmer control. The physician ca select theparticular interval that is used to determine whether or not a givencardiac interval can be classed as a TACH A, a TACH B or a fib interval.It is those programmable parameters, i.e., the interval cutoff and thedepth of the bin, that are used in the implementation of detectionhysteresis. Thus there are two parameters per arrhythmia in thisembodiment.

Typical examples for those parameters are set forth above. For example,TACH A may be set at 450 msec., TACH B at 380 msec., and fibrillation at310 msec. Thus, if the system detects an interval that is between 450msec. and 380 msec., it would be binned as a TACH A interval if theaverage were also TACH A. If the interval were between 380 msec. and 310msec., it is a TACH B interval if the average is TACH B or TACH A. Ifthe interval is shorter than 310 msec., then it is binned as afibrillation interval (as long as AVERAGE is shorter than TACH A; if itis not, then no interval is binned). If the interval is longer than 450msec., then it is a sinus interval. The number of intervals required foreach diagnosis may be 3 to detect sinus, 8 to detect TACH A, 8 to detectTACH B and 12 to detect fibrillation. This means that the fibrillationbin is 12 deep, the sinus bin is 3 deep, etc. The bins, however, can beprogrammable up to 255 intervals but the previous numbers are beinggiven a specific examples.

As a further explanation of the extended high rate (EHR) system, as soonas the average goes from being a sinus average to a EHR average (TACH Ain this example), the EHR timer is started. Then, if the EHR timer timesout at any time, whether the bins are full or not, that determines thatthe system has detected EHR. EHR is another way to start fibrillation orsome other programmed therapy. If the system detects sinus at any time,or if it detects fibrillation or EHR at any time, the EHR timer iscleared.

The following is an example of what will happen if any arrhythmia startsthe EHR timer. If the arrhythmia does not persist, and sinus rhythm isdetected, then the EHR timer is cleared until another arrhythmia starts.The arrhythmia starts, TACH A is detected, some TACH A therapy is given,and possibly TACH B is detected and some TACH B therapy is given. TheEHR timer then times out, and the system will abandon the TACH A or TACHB therapy and will revert to fibrillation therapy. In other words, it isbasically a safety exit to prevent the patient from being engaged inless effective therapies for a long time.

The interval alternans flow chart of FIG. 5 will now be discussed.

The interval alternans section is entered from the determine rhythmblock. The average is checked to see if it is greater than or a TACH Baverage. If it is not greater than TACH B, then the rhythm is rapid.Thus it is either a TACH B average or a fibrillation average and in thatcase therapy is delivered whether or not there is an interval alternans.

If, however, the average is greater than a TACH B interval, that meansthat the patient has a TACH A average and then a further check is madeto see whether there have been more TACH intervals than sinus intervalsduring the detection period. If there have not, then the systemdetermines that it is not an arrhythmia and awaits the next R-wave. Ifthere have been more TACH or fibrillation intervals than sinusintervals, the system continues on to deliver the therapy. The reasonfor this is that if there is a bigeminal rhythm, the average rate mightbe a TACH A rate but the patient is not in arrhythmia and it isdesirable to avoid treating the patient, even though the short intervalsmay be TACH B intervals or fibrillation intervals. This intervalalternans program provides a means to avoid treating a bigeminal rhythm.

A novel system for cardiac therapy has been shown and described. Bymeans of the present invention, the system can rapidly converge to asolution and in the event that the arrhythmia is indeterminate, there isa default to the most conservative diagnosis. By using a novel method ofbinning, the system allows the use of a higher arrhythmia detection ratein a tiered defibrillator, without the disadvantage of failing toterminate slower arrhythmias that might result from the therapy. Theinvention also allows the system to keep track of the ratio of sinusintervals to tachycardia intervals, and to require more tachycardiaintervals than sinus intervals so that a bigeminal rhythm will not bedetected as a tachycardia.

Although a detailed explanation of the invention has been shown anddescribed, it is to be understood that various modifications andsubstitutions may be made by those skilled in the art without departingfrom the novel spirit and scope of the present invention. In particular,incrementing as herein used is meant to include counting up to a presetlimit, or counting down from the limit, or any other manner ofmaintaining bins electronically.

What is claimed is:
 1. A cardiac therapy method using an implantedcardiac pulse generator, which comprises the steps of:sensing apatient's heartbeat; determining the intervals between heartbeats; if atachyarrhythmia is detected, then starting a duration timer to time apredetermined time period; if sinus rhythm is detected during saidpredetermined time period, then clearing the duration timer; iffibrillation is detected during said predetermined time period, thenclearing the duration timer and treating for fibrillation; and if atachyarrhythmia other than fibrillation continues for said predeterminedtime period, then commencing therapy.
 2. A method as defined in claim 1,wherein the step of determining the intervals between heartbeatsincludes the step of averaging a selected number of intervals.
 3. Amethod as defined by claim 1, wherein therapy commences after timeout ofsaid duration timer.
 4. A method as defined by claim 1, wherein saidtherapy comprises the step of applying a high energy shock to thepatient's heart.
 5. A cardiac therapy method using an implanted cardiacpulse generator, which comprises the steps of:sensing a patient'sheartbeat; determining the intervals between heartbeats, including thestep of averaging a selected number of heartbeats; if a tachyarrhythmiais detected, then starting a duration timer to time a predetermined timeperiod; if sinus rhythm is detected during said predetermined timeperiod, then clearing the duration timer; if fibrillation is detectedduring said predetermined time period, then clearing the duration timerand treating for fibrillation; said treatment for fibrillationcomprising the applying of a high energy shock to the patient's hear. 6.A cardiac therapy method using an implanted cardiac pulse generator,which comprises the steps of:sensing a patient's heartbeat; providingstorage means including a plurality of storage bins, each of whichcorresponds to a different cardiac rhythm band; determining theintervals between heartbeats; incrementing the storage bin correspondingto the cardiac rhythm band of the determined heartbeat interval;assigning a maximum count limit to each storage bin; detecting when afirst bin of said plurality of storage bins reaches its maximum countlimit; providing a diagnosis of the patient's cardiac rhythm that isresponsive to the first bin to reach its maximum count limit; if atachyarrhythmia is detected, then starting a duration timer to time apredetermined time period; if sinus rhythm is detected during saidpredetermined time period, then clearing the duration timer; iffibrillation is detected during said predetermined time period, thenclearing the duration timer and treating for fibrillation; and iftachyarrhythmia other than fibrillation continues for said predeterminedtime period, then commencing therapy.
 7. A cardiac therapy system whichcomprises:an implanted cardiac pulse generator including means forsensing a patient's heartbeat; means for determining the intervalbetween heartbeats; a duration timer for timing a predetermined timeperiod if a tachyarrhythmia is detected; means for clearing the durationtimer if sinus rhythm is detected during said predetermined time period;means for clearing the duration timer and treating for fibrillation iffibrillation is detected during said predetermined time period; andmeans for commencing therapy if tachyarrhythmia other than fibrillationcontinues for said predetermined time period.